The invention relates to a traveling field tube particularly a high-efficiency traveling wave tube having a magnetic system for the bundled guidance of an electron-beam, which system surrounds a delay line comprising line cells arranged one behind the other, separated from one another by transverse walls, each of which is provided with an opening therein for the passage of the electron beam, and a coupling opening therein extending substantially in circumferential direction, with the delay line coupled, at least at one of its two frontal sides with a rectangular hollow conductor whose broad side adjacent the longitudinal axis of the delay line extends perpendicularly thereto and which conducts at a right angle with respect to the line axis.
Traveling wave tubes constructed in this general manner are known, for example, as illustrated in British Pat. No. 953,488 or German Offenlegungsschrift No. 2,102,230 (see FIG. 1 in conjunction with page 4, third paragraph thereof). In such type of tubes, the rectangular hollow conductor usually is provided with a metallic projection or protuberance which extends longitudinally along the inner wall surface of the hollow conductor opposite to that disposed adjacent the delay line with such structure very closely approaching the delay line adjacent the opening for the passage of the electron beam (which approaches in steps in the cited Letters Patent and in a continuous manner in the cited Offenlegungsschrift) and cooperates with a short circuited line section of high impedance. Such a longitudinal path loading provides a good junction between the comparatively high wave resistance of the delay line and low wave resistance of the hollow conductor and enables a broad-band, low-reflection HF-coupling and decoupling.
However, the favorable matching values are offset by several properties or characteristics which become disadvantageously noticeable where higher powers and higher frequencies are involved, and in particular where a permanent-magnet system is utilized for the spatial periodic bundling of the electron beam (PPM System). Such path loading comprises, for example, edge and corner configurations in the direct area of the frontal coupling opening and electron-beam passage opening of the delay line, which may create high field-strength concentrations and result in arcing when the hollow conductor is employed for energy decoupling. In addition, loaded hollow conductors require dimensions, particularly in the direction of the line axis, which would create inadmissably large interference of the electron bundling in the case of such a permanent-magnet system (PPM).